The search for alternate energy sources in the United States and throughout the world is progressing at an ever increasing rate as the available supplies of energy are being consumed. There are many alternate sources of energy which might be tapped but for technological and/or cost considerations. Solar energy is one source which is being extensively examined due to its abundance and to an apparent absence of environmentally deleterious side effects.
The technology and theory for producing basic photovoltaic cells which generate electrical energy in response to solar input is generally well known. The main technical problems which are currently under investigation deal with reducing this basic technology to a practice which is applicable to the production of such photovoltaic cells at a cost which is competitive with that required to construct and operate present day power generating facilities utilizing such energy sources as oil, coal, or nuclear fission. To accomplish this goal, it is apparent that electrical generating stations utilizing photovoltaic cells must be fabricated using mass production techniques wherein large areas, measured in terms of square miles, can be literally covered with such mass-produced photovoltaic cells. In accordance with the present invention, large area photovoltaic panels will be formed using production-type techniques and will thereafter be formed into an array of series connected photovoltaic cells in a process suited to mass production and sized to generate commercial quantities of electrical energy.
One technique for producing photovoltaic cells with polycrystalline CdS is to spray suitable solutions onto a substrate where the solution reacts to form a film of the desired material. U.S. Pat. No. 3,880,633 and No. 3,902,920 to Jordan et al disclose suitable techniques for forming large area back-wall type photovoltaic cells by the spray method. A glass substrate is moved through a series of spray booths to form layered films of tin oxide, cadmium sulfide, and perhaps cuprous sulfide. It is a feature of these spray processes that each film is formed at a temperature lower than that at which the preceding film is formed. Accordingly, it would be desirable to form the large photovoltaic panel into some number of smaller cells, to be connected in series for increased voltage outputs, only after all of the layers have been formed which require heat input to drive the reaction forming the layer. Such a technique would minimize the thermal cycling of the glass and the energy required to produce the photovoltaic panel.
U.S. patent application Ser. No. 831,544, a related case whose disclosure is hereby incorporated herein by reference, fully discusses various prior art techniques for interconnecting pluralities of photovoltaic cells whereby useful quantities of electrical energy can be made available. The subject application further discloses a backwall array of series connected photovoltaic cells having substantially continuous electrode areas for collecting electrical currents generated by the cells and having adjacent extending surface areas forming the desired interconnections.
A particular embodiment of the backwall array is formed to include a transparent vitreous substrate receiving incident radiation; a transparent conductive layer, which may be tin oxide (SnO.sub.x): a semiconductor layer, which may be cadmium sulfide (CdS); a heterojunction, which may be copper sulfide (Cu.sub.x S), forming a barrier layer; and one or more layers of an upper conductor material, such as metallic copper coated with a protective layer of lead. Separation of the upper conductor to obtain the series connection is obtained by removing a strippable material deposited prior to depositing the conductive materials.
It has been found, however, that the adherence between the tin oxide layer and the copper layer provides only a low resistance to forces tending to separate the two layers. Further, the copper sulfide forming the heterojunction is exposed once the strippable material is removed. In some instances, the electrical field adjacent the exposed copper sulfide can act to convert the copper sulfide to metallic copper which lowers the shunt resistance between cells, thereby reducing the overall conversion efficiency of the array. Yet another possible degradation can occur when the Cu.sub.x S layer is formed prior to component layer removal for cell separation. In this instance, the Cu.sub.x S layer may be exposed to the atmosphere for a time sufficient to form copper oxide in amounts effective to degrade overall cell, and hence array, performance.
The disadvantages of the prior art are overcome by the present invention, however, and improved methods are provided for obtaining an array of photovoltaic cells connected in series. Further, an improved array of series connected photovoltaic cells on a common substrate is provided.